The present invention relates to a method for operation of a nuclear reactor of boiling water type with a reactor core comprising a plurality of vertical fuel assemblies of substantially square cross section, arranged in a lattice with each fuel assembly included in two rows of fuel assemblies, perpendicular to each other and separated by intermediate water gaps, and wherein each one of two vertical sides of a fuel assembly, which are perpendicular to each other, is facing towards a water gap which has a larger width than the water gap towards which its opposite vertical side is facing. Each fuel assembly contains a plurality of vertically arranged fuel rods. The core also contains a large number of control rods, which may each comprise four vertically arranged blades provided with a neutron absorber, which blades form a perpendicular cross and which are adapted to be insertable into the wide water gaps between the fuel assemblies but not into the narrow water gaps.
When the burnup in a reactor of the above-mentioned kind has progressed so far that the smallest acceptable core reactivity margin has been attained, a partial recharge is carried out. By balancing in a suitable way how much fuel that is to be replaced as well as the enrichment of the replacement fuel, an excess reactivity is obtained which permits a certain energy output until the next refuelling. During the partial recharge, one-fifth of the fuel, for example, can be replaced every operating year (or any other suitable operating cycle) as from the end of the second operating year. This means that the fuel in the exemplified case remains in the core for five years during steady state, but that part of the fuel which is replaced during the initial stage is used for a shorter period of time.
Refuelling is performed in such a way that fuel assemblies are withdrawn from the core and fuel assemblies with fresh fuel, usually after appropriate relocation of remaining fuel assemblies within the core, are inserted into the empty spaces arising. The relocation of fuel assemblies is carried out in order to achieve optimum power distribution within the reactor core and optimum reactivity.
In reactors of the kind stated, the enrichment distribution in the fuel assemblies is normally uneven. High enrichments, that is, fuel rods with a high content of fissile material, are placed along the narrow water gap, where the moderation is poorest and low enrichments, that is, fuel rods with a low content of fissile material, along the wider water gap, where the moderation is good. This is necessary for the performance with respect to the thermal margins to be good when the fuel assembly is relatively fresh and has a high power. By thermal margins are meant in this case primarily margins with respect to dryout, that is, with respect to powers where there is a risk of water films breaking down on fuel rods in the fuel assembly, and margins with respect to powers where there is a risk that the linear power density of the fuel rods gives rise to damage on the cladding which surrounds the nuclear fuel material as a consequence of the occurrence of swelling of the nuclear fuel material.